Acoustic pressure in the enclosed cavity affected by particle damping was firstly studied. By inducing the formula of acoustic pressure, the contact between modal shapes and acoustic pressure was studied. The modal shapes of the enclosure cavity which is simplified and reduced proportionally from a vehicle body were analyzed by modal test. By equipping the board of which relative displacement is the maximum in lower-order modal shapes of the enclosure cavity with particle damping, the vibration of the lower-order modal is reduced. Then the acoustic pressure of the enclosure cavity is reduced. The results show that compared with none particle damping, the decrease of the A-weighted RMS acoustic pressure at the reference point in the enclosure cavity is the most, from 104.56dB to 101.03dB, decreased by 3.4% when particle damping was equipped on the board of which relative displacement is the maximum in first modal shape of the enclosure cavity.